fix(avatars): integrate the profile images backend

We shouldn't merge this before the dev backend is ready: https://github.com/dannycoates/fxa-dev/issues/24.

Fixes #1556.
For testing this, it would be cool to mock out the `OAuthClient`, `ProfileClient`, and `Assertion` module and check that the expected methods on those objects are called with the expected parameters (e.g., the expected audience to `Assertion.generate`). In general, this is how we'd like to use mocks to test our objects, and I think this would be a good trial of using that approach (i.e., mocking out the direct object dependencies and not necessarily the actual servers).